The manufacture of integrated circuits on semiconductors involves several steps. Such steps include growing a semiconductor material, slicing the semiconductor material into planar structures, and polishing the surface of the semiconductor. Once the semiconductor is polished, circuit elements may be formed by selectively introducing impurities, or dopant, into portions of the semiconductor. Impurities alter the electrical behavior of the semiconductor in predictable ways to create various circuit elements. One method of introducing impurities into the semiconductor is through ion implantation.
Ion implantation is a process in which impurities are implanted into the semiconductor substrate to form circuit elements within the semiconductor crystalline structure. In a typical ion implantation process, various parts of the substrate are masked with a pattern that is made of photoresist or another suitable material that substantially prevents the ions from penetrating. The portions of the silicon in which the mask is absent, i.e. where voids in the mask are present, absorb the ions during implantation. In this manner, impurities may be selective implanted within the substrate.
In general, the ion implantation process itself involves multiple steps. In particular, a mask must first be designed in accordance with the circuit layout. Thereafter, the silicon and mask are bombarded by ions. The mask is then removed in a suitable manner, many of which are known in the art.
Many integrated circuits employ different circuit components that require different concentrations of dopant. To form such different concentrations, the ion implantation step may be repeated for every different level of doping concentration required in the integrated circuit. For example, a first mask may mask all but the areas requiring a high level of doping concentration. Once the mask is applied to the surface, the ion implantation device then bombards the silicon and mask at the highest level. After implantation and removal of the first mask, a second mask is applied to the surface to mask all but the areas requiring a lower level of doping concentration. Ion implantation then takes place at the lower level.
While the above method can reliably implant dopant into a silicon substrate at multiple concentration levels, the manufacturing process is extended in substantial part by the repeated ion implantation operations. For example, if a particular integrated circuit requires four or five different dopant concentration levels, the repeated masking and implantation steps significantly lengthen the manufacturing process.
There is a need, therefore, with a method of implanting impurities into semiconductor substrates at multiple levels that has reduced processing time.